1. Field of the Invention
The present invention relates to a process for the preparation of a magnetic recording medium improved in running endurance and a different grinders preferably, employable for the same.
2. Description of Prior Art
A magnetic recording medium comprising a nonmagnetic support and a magnetic recording layer provided thereon is used as an audio tape, a video tape, or a recording medium for a computer system. The magnetic recording layer comprises a binder and a ferromagnetic powder dispersed therein.
The magnetic recording medium is generally prepared by the following process. A binder such as a resin component and a ferromagnetic powder are dispersed in a solvent to prepare a magnetic paint, and the magnetic paint is coated over a nonmagnetic support to form a layer of the magnetic paint on the nonmagnetic support. The coated layer of the magnetic paint is then subjected to various treatments such as magnetic orientation, drying treatment and surface smoothening treatment. Subsequently, the sheet having been subjected to such treatment steps is cut or slit into a desired shape to prepare a desired magnetic recording medium.
It is though that the particles of the ferromagnetic powder are firmly kept in the recording layer of the magnetic recording medium prepared as above and the magnetic recording layer has a very smooth surface. According to studies of the present inventors, however, it has been confirmed that some particles of the ferromagnetic powder are insufficiently fixed to the recording layer and merely deposited on the surface of the recording layer. These insufficiently fixed ferromagnetic particles easily separate (drop off) from the recording layer and deposit on the surface of a magnetic head in the course of running procedure, to cause clogging on the head. Further, these particles sometimes cause occurrence of drop-out in the case of a video tape, etc.. Moreover, such separation of the ferromagnetic powder from the recording layer reduces the amount of the ferromagnetic powder present in the vicinity of the surface of the magnetic recording layer, so that the resulting magnetic recording medium is lowered in the electromagnetic conversion characteristics (i.e., lowering of output level occurs) after repeated uses.
For solving the above-mentioned various problems such as occurrence of drop-out, clogging on a magnetic head and lowering of output level, the present inventors have invented a method of grinding the surface of a magnetic recording layer by means of a grinder and applied for a patent with respect to the method (U.S. Pat. Ser. No. 007,216).
According to the method, the surface of the magnetic recording layer which has been subjected to a surface smoothening treatment is ground by means of a grinder of high hardness such as a diamond wheel or a fixed sapphire blade to remove particles of the ferromagnetic powder easily dropping off from the recording layer or material deposited on the surface of the recording layer, so as to prevent separation of those particles and material from the recording layer in the course of running procedure. By grinding the surface of the magnetic recording layer as described above, various disadvantages such as occurrence of drop-out or clogging on the head and lowering of output level after running of the medium for a long period of time are effectively prevented.
A magnetic recording medium may have a back layer (i.e., backing layer) comprising a binder and a nonmagnetic powder dispersed therein on the free surface of the nonmagnetic support (surface on which the magnetic recording layer is not provided) for improving running properties. The present inventors have also confirmed that the magnetic recording medium having a back layer tends to increase in occurrence of drop-out or clogging on a magnetic head as compared with a magnetic recording medium having no back layer, though the medium shows almost the same level as that having no back layer in lowering of output after repeated running by grinding the surface of the magnetic recording layer. That is, as for the magnetic recording medium having been ground on the surface of the magnetic recording layer, the back layer provided for improving running endurance does not function satisfactorily from the viewpoints of reduction of occurrence of drop-out and prevention of clogging on a head.